When oxygenous compounds are separated by distillation, a high energy consumption must be taken into account. In addition, for instance, with the recovery of fermentation products from fermentation mashes, the temperature load during distillation must be considered, which cannot be totally avoided even by cumbersome vacuum distillation. The heating of mashes to elevated temperatures, however, prevents the mashes, which have largely been freed from fermentation products, from being recycled into the fermentation stage.
Yet, it is the separation of the initially mentioned compounds which is of a major importance to the recovery of the products of fermentation processes from aqueous fermentation media:
So far, relatively dilute mashes, as a rule, have been used for fermentation, for instance, with the large-scale production of ethanol by alcoholic fermentation, because the fermentation product ethanol has a toxic effect on the microorganisms used for fermentation already from concentrations of about 5%, which inhibits fermentation. The fermentation process mostly is carried out discontinuously, processing of the fermented mash being effected by distillation. As a result of the high mash dilutions required, large amounts of water must be conducted through the individual process stages with a correspondingly high consumption of energy involved.
By advances in raw material processing and distillation, the energy demand will be lowered only slightly.
Therefore, there has been the endeavor to attain a further increase in productivity and a further decrease of energy demand by employing concentrated mashes and fermentation media as well as by carrying out fermentation continuously. Yet, concentrated mashes can be fermented only if the concentration of the fermentation products--such as ethanol--is maintained below the inhibiting threshold during fermentation.
Basically, the direct removal from the fermentor, such as, e.g., by vacuum fermentation or by CO.sub.2 stripping, or the separation by circulation of the mash through an external separating system are feasible to separate the product from the fermentation mash during fermentation.
The realization of vacuum fermentation raises problems with regard to fermentation control, in particular with technical plants, moreover, the subsequent compression of the carbonic acid formed is uneconomical.
The separation of the fermentation product by stripping with CO.sub.2, so far, has been tested primarily on a laboratory scale; with the separation performances required, disturbances of fermentation may occur.
As external separation systems, adsorption, extraction, membrane methods or methods employing liquid-steam enrichment may be used.
As extraction agents for liquid-liquid extraction, most of the current solvents have already been tested. Yet, the solvents in question have relatively high solubilities in water or in the aqueous fermentation medium and mostly have strongly toxic effects on the microorganisms used for the fermentation.
The same considerations with respect to substrate concentration in fermentation liquors and to economical fermentation control hold for other types of fermentation, for instance, butanol-acetone fermentation.
In DE-A No. 31 12 603, a three-stage process for the separation of lower aliphatic alcohols from fermentation liquors is described. The only recoverable fermentation product expressly mentioned is ethanol.
In the first stage, the fermentation liquor preferably is extracted with a higher n-alcohol. To recover the residual content of the solvent used in the first extraction stage from the aqueous raffinate phase obtained, the raffinate phase, in the second stage, is extracted with an a polar second solvent, in particular with an alkane having 5 to 18 carbon atoms. The two solvents consecutively used for the extraction again must be separated from each other in a third stage.
To realize the known process, two extraction devices and two separating devices are necessary. Thus, this process requires extensive energy and apparatus means. Add to this that the extraction is to take place at system temperatures as high as possible, up to 60.degree. C.